1. Field of the Invention
This invention relates to optical devices in which a semiconductor quantum well structure is formed as an asymmetric dual quantum well (ADQW) structure including a plurality of different quantum wells (typically two wells) for achieving a specific property. According the specific property, only a refractive index in the ADQW is changed, but not an absorption factor therein, by applying a reverse bias electric field thereto.
2. Related Background Art
In recent years there has been much interest in exploiting and developing optical communication systems which are capable of transmitting a great capacity of information at a high speed. Among them, wavelength or frequency division multiplexing systems and coherent optical communication systems are highlighted for their capability of communicating a greater capacity of information at a high speed which can be obtained by making a use of characterictics of light. Therefore, the intense effort has been made in developing devices for processing lights of different wavelengths or for modulating the phase of Light and the like.
In the wavelength division multiplexing systems, if semiconductor lasers are used as a light source, a wavelength range of tunable or changeable radiation lights is at most several tens .ANG., so that when the wavelength division multiplexing communication is conducted within such wavelength range, devices for selecting or filtering light having a wavelength spectrum width less than 1 .ANG. at a resolution of 2.about.3 .ANG. are required.
FIG. 1 shows the structure of a waveguide type filter including a distributed reflector portion which is an example of the above-mentioned devices (see Numai et al.: Tunable filter using a phase shift type DFBLD, Informal Paper No. C-161 distributed at Autumnal Grand Meeting of Electronics Information Commun. Academy (1988)).
In the device of FIG. 1 of the distributed feedback laser diode (DFBLD) type, there are provided, on a substrate 201, a waveguide 202 with a grating 203 and a partial active layer 204. The device is divided into active portions 205a and 205b including the active layer 204 and a phase regulating portion 206 without the active layer 204 formed therebetween. Anti-reflection coats 207a and 207b made of SiN.sub.x are deposited on both end surfaces of the device. Reference letters N.sub.a, N.sub.p and .theta. respectively denote refractive indices of the waveguide 202 in the active portions 205a and 205b and the phase regulating portion 206 and the length of the phase regulating portion 206. The refractive indices N.sub.a and N.sub.p in the waveguide 202 are changed by injecting an active current I.sub.a and a phase regulating current I.sub.p into the respective portions 205a, 205b and 206 to vary carrier densities in the waveguide 202 thereat. Thus, the wavelength of light reflected by the grating 203 or distributed reflector structure is varied to achieve a tunable filter.
Such a tunable filter, however, has drawbacks that a decrease of the refractive index resulting from the carrier injection into the waveguide is reduced by an increase of the refractive index resulting from a rise in temperature due to current flowing in the waveguide 202 and that thermal noise also occurs due to such rise of temperature. Moreover, there are several disadvantages that spontaneous emission appears due to the carrier injection, that a beat noise occurs from interaction between the spontaneous emission and an incident light, and others.
FIG. 2 shows another prior art example which is a filter of Fabry-Perot type using a structure similar to surface emission type lasers (see Kubota et al.: Tunable filter of surface emission laser type, Informal Paper No. C-246 distributed at Vernal Grand Meeting of Electronics Information Commun. Academy (1990)).
In the device of FIG. 2, there are provided, on a substrate 211, an etch-stop layer 212 and layers 213, 214, 215 and 216 of n-InP, p-GaInAsP, p-InP and p-GaInAsP, Anti-reflection coats 217 and 218 of TiO.sub.2 /SiO.sub.2 multi-layered film are respectively formed on the layer 216 of p-GalnAsP and the etch-stop layer 212 in a window etched in the substrate 211. In this device, a selected or filtered wavelength of output light is changed by varying the refractive index by the injection of carriers, similar to the device shown in FIG. 1.
Such filter, however, also has drawbacks similar to those of the device of FIG. 1 because carriers are injected into the device to shift the selected wavelength. That is, characteristics are degraded under adverse influences of the rise in temperature and spontaneous emission, similar to the device of FIG. 1. As a result, a tunable width of wavelength becomes narrow.
FIG. 3 shows a prior art example which is a phase modulating device in which an electric field is applied to a superlattice layer 223 (intrinsic multiple quantum well (i-MQW) layer) to change the refractive index thereof by quantum confined Stark effect (QCSE) (see H. Nagai et al.: Field-induced modulations of refractive index and absorption coefficient in a GaAs/AlGaAs quantum well structure, Elect. Lett. 14th vol. 22, No. 17 (1986)).
In the device of FIG. 3, there are formed, on an n-GaAs substrate 221 (dopant concentration: 4.times.10.sup.18 cm.sup.-3), an n-Al.sub.0.4 Ga.sub.0.6 As layer 222 (thickness: 35 .mu.m and dopant concentration: 1.times.10.sup.16 cm.sup.-3), the i-MWQ layer 223 and an electrode 224 of Au thin film (thickness: 200 .ANG.). In the substrate 221, a window is etched and another electrode 225 of Au thin film (thickness: 200 .ANG.) is formed. The i-MQW layer 223 is comprised of ten periodes of GaAs layer (thickness: 100 .ANG.) and Al.sub.0.4 Ga.sub.0.6 As layer (thickness: 150 .ANG.).
Such phase modulating device, however, also has the following drawbacks. Since an exciton wavelength of the i-MQW layer 223 is shifted to a longer one or greater value due to the QCSE by the applied electric field, absorption factor thereof as well as refractive index is changed and hence the intensity of an output varies if the wavelength of an input light is changed. Thus, a usable width of wavelength of the input light becomes narrow. Further, the absorption factor abruptly increases near the wavelength of light, to be phase-modulated, so that the intensity of output light decreases greatly.